The invention relates generally to synchronous machines having round or salient pole rotors.
Most synchronous machines are designed to include either permanent magnets or excitation windings (fed by a regulated source) to provide MMF (magnetomotive force) that provides the magnetic flux for machine operation. The permanent magnets or excitation windings can be situated on either the rotor or the stator. For rotor-based embodiments, excitation power is brought through either a set of collectors (slip rings) or a brushless system that uses a small (xe2x80x9cinside-outxe2x80x9d) synchronous machine with a stator excitation source.
Permanent magnet machines are not easily regulated. Operation of permanent magnet machines in constant power mode can be a problem because low power factor operation is forced through flux weakening methods to reduce voltage at light loads or to minimize inverter ratings. As a result, the machine terminal voltage becomes load dependent, efficiency suffers, and, at partial loads, the magnetic field source is underutilized.
Machines wound with excitation windings (xe2x80x9cwound field machinesxe2x80x9d) can be regulated over a wide range of loads, but wound field machines experience winding losses that decrease machine efficiency. Additionally, windings and excitation sources for wound field machines are sized to support the maximum requirements and thus are often expensive and under-utilized.
Evans et al., U.S. Pat. No. 5,663,605 describes a salient pole alternator for automotive applications including a rotor having both wound-field and permanent magnet poles disposed about the circumference of the rotor. The magnets are situated inside salient poles, and the windings are situated around different salient poles from the magnet poles. Automotive alternators operate at low magnetic and mechanical stress levels and often include complex and fragile multiple pole configurations. For applications requiring higher power machines, such as boiler feed pumps and utility generators, for example, more practical configurations would be desirable.
It would therefore be desirable to have a durable machine that further combines the advantages of the excitation control of wound field machines and the advantages of higher efficiency of permanent magnet machines.
Briefly, according to one embodiment of the present invention, a hybrid machine comprises a cylindrical element having slots; excitation windings situated in at least some of the slots; and permanent magnets situated in at least some of the slots.
According to another embodiment of the present invention, a hybrid machine comprises a cylindrical element having salient poles; excitation windings situated around the salient poles; and permanent magnets supported by the salient poles.